fix: keep entity labels optional in decision v2

[abrichr]

What changed

• Keeps the workflow entity-class label optional presentation metadata.
• Derives the neutral fallback from the selected reviewed label, so the CLI no longer asks the operator to make a redundant choice.
• Emits a signed V2 decision task with neutral record wording when a V2-capable peer receives a task for a certified step that has no domain label.
• Adds contract coverage that proves label absence does not block certification.

Why

The entity label improves operator wording. It is not identity evidence, effect evidence, or a production admission requirement. A missing optional label must not downgrade a V2 decision task and discard its stronger qualification, revision, contract, and step bindings.

This pull request is stacked on draft PR #308. It does not change the Standard or Regulated qualification requirements.

User impact

The workflow owner can leave the optional label unset. The operator then sees neutral record wording. If the owner selects a reviewed label, Flow derives the correct neutral fallback automatically.

Checks

• Ruff on the five changed files.
• 186 tests in tests/test_qualification_project.py and tests/test_attended_actions.py.
• git diff --check.